Sonar (for SOund NAvigation and Ranging) is a technique that uses sound propagation (usually underwater) to navigate, communicate or to detect other vessels or objects. There are two kinds of sonar—active and passive. Active sonar pulses sound waves through a medium (e.g., water) and receives waves reflected by the other vessels. Passive sonar only receives sounds waves propagated by the other vessels. Sonar may be used as a means of acoustic location. The term sonar is also used for the equipment used to generate and/or receive the sound waves. The frequencies used in sonar systems vary from infrasonic to ultrasonic. Detection of the other vessels with sonar (and particularly passive sonar) is inherently difficult. The primary difficulty is the variation of the received acoustic signal resulting from a complex propagation structure of the medium (e.g., an ocean) through which sound waves are transmitted. The received signal may, for instance, have transitioned through a spectral region of elevated broadband noise produced by a noisy surface vessel.
Detection of a sonar contact by standard spatial and time domain processing to optimize the target signal-to-noise ratio can be employed in passive sonar systems. An estimation of target position and velocity from passive sonar bearings can be referred to as bearings-only target motion analysis (TMA). Signal detect-before-track methods are intended to produce a reliable track of sonar contact. Unfortunately, detect-before-track methods are limited by the integration time consistent with a sonar contact's kinematics. A computed acoustic power at bearings over a sonar system's field-of-view and low signal-to-noise ratios are insufficient for reliable TMA solutions. Some techniques have been proposed to increase the integration time. These techniques increase the integration time by combining detection and localization operations into track-before-detect algorithms. In such cases, TMA methods are still employed, usually requiring a search over a large number of hypothesized target tracks and finding the hypothesized track that maximizes a long-term integrated power computed for each hypothesized track.